Randall RG 235 DS schematics low output

[nosaj]

I just picked up a randall RG 235 DS head and 2 stereo spks for $60 output seem audibly low.  Does anyone have a schematic for it?

Caps are first thing to be replaced but just wanted to check values as I see some mods in there.

Thanks,
nosaj

[J M Fahey]

Stop random unguided parts replacement until you have a real problem to solve and have found some cause for it.

"Low power" is too generic.

[Roly]

Caps are first thing to be replaced

FFS WHY???

"Scattergun" servicing doesn't work - that's why professionals don't do it that way.

Measurements, diagnosis first, then fix what is actually wrong.  It's always quicker and cheaper.

[nosaj]

Was under the impression that electrolytics 20 years + should be replaced.

ok here's what I've found since yesterday.

on normal channel maxed audio with audio pumped in, on stereo output one measure .06vac in 8 ohm resistor.
on overdrive channel maxed audio with audio pumped in, on stereo output one measure 10vac in 8 ohm resistor.

Looking near inputs on board is a rc4558p opamp which I'm guessing handles normal and overdrive.

Output transistors are lm1875t  so it looks like 20 watts max?

Still in process of finding a schematic, near the rc4558p chip there are 2 leads which look like a component was clipped out.

Thanks,
Jason

[Roly]

Was under the impression that electrolytics 20 years + should be replaced.

This comes under the heading of "Popular Mythology".

50 years maybe, but at 20 years I'd still want to have a good reason such as dented/leaking/bulging, high ESR, low capacity.

If you really must, there is also a good reason to leave such sweeping replacements until after the amp has been repaired, and it never gets a mention in the forums that advise "jump in and replace everything".

The process of replacing a component also carries the risk of making thing worse in some way (e.g. putting one of the new caps in backwards as has happened here) and introducing a second fault to an already faulty circuit, and faults don't add in complexity, they multiply, so randomly replacing a bunch of components will very often muddy the waters and make a fairly simple faultfinding job more difficult, particularly for the less experienced who may already be stretched by a single fault.

Output transistors are lm1875t  so it looks like 20 watts max?

This is actually an integrated chip amp, and you'll find the datasheet here.

near the rc4558p chip there are 2 leads which look like a component was clipped out.

Ah HA!  Well observed.  :tu:  This could be a failed mod (or it could be nothing).  We need a crisp well lit pic of this section of the board.  Also if you could trace the connections on the copper side of the board and see if they go to one or more of the pins of the op-amp, and if so, which ones?

[nosaj]

Will get those pics shortly. Just took measurements on rc4558p by input 
pin1 .13 vdc
pin2  .16 vdc
pin3 no voltage loud sqeel
pin4 -11.3
pin5 loud hum like touching a grid input on a tube
pin6 smaller hum like above
pin7 no measurement
pin8 10.5 vdc

Output transistors lm1875t
left channel
pin5 23.9
pin3 -16.1

right channel
pin5 24.1
pin3 -24.1

Pics attached. Looks like it ties the 2 ground planes together ??

[nosaj]

There are several of the rc4558p chips so I switch one that looked to be in the chorus section for one near the input that is in the pictures.  No change. What I'm wonder now is if the normal channel and the overdrive channel are on the same rc4558 .  Normal uses one half of the rc4558 and overdrive uses both?  Either way normal channel vol seems way too low.
Tomorrow when i get a chance I'll start with the power supply where I should have started first. Cause if it isn't right nothing else will be.

Will update tomorrow if possible.

jason

[DrGonz78]

Quick question...?? I can find no information about any Randall RG235 but there is a RC235 stereo chorus amp. Perhaps that is what you meant?

[nosaj]

Yes rc235 ds.

Sorry

[nosaj]

Here's the schematic Randall sent it today.

[nosaj]

ok power supply voltages look correct  24.7 +-  and 11.8 +-.

So the 25v and 12v rails look ok.

Amazing what happens when you loosen all the connections then put it back together.

Everything seems to be working just fine now. All that was done was clean pots and jacks. removed circuitboard( which meant loosing pots), taking measurements then putting it back together.

Jason